1. Field of the Invention
The invention relates to a contact system for gas blast circuit breakers with a nozzle-shaped fixed contact piece; a second, coaxially mounted nozzle-shaped, axially movable contact piece; and a movable bridging contact piece common to both.
2. Description of the Prior Art
A contact system of this type is disclosed in Canadian Pat. No. 1,035,817 (which corresponds to German Auslegeschrift DE-AS No. 24 44 943), wherein one nozzle-shaped contact piece, from which the bridging contact rolls off, is axially movable against the force of a spring in the direction of another nozzle-shaped contact piece which is fixed, and is connected with the secondary coils of two electrodynamically acting coil systems.
In the known gas blast circuit breaker, the switching work is reduced in that electrodynamic control of the roll-off switch element is achieved. This is done in that at sufficiently high currents the switch switches quasi-synchronously. During the break process, the current to be broken, flowing in a first coil system, initially produces a high repulsive force, by which the roll-off contact piece is accelerated in the direction of the other nozzle-shaped element, so that the gap is reduced. Shortly before the current to be broken passes through zero, this force changes to an attracting force, increasing the gap, because of the specifically chosen phase angle in the first coil system. Consequently, the necessary quenching distance between the two nozzle-shaped contact pieces is achieved during passage through zero.
It is further known from U.S. Pat. No. 4,258,239 (which corresponds to German AS No. 27 03 550) that a second nozzle-shaped contact piece can be associated with a first fixed nozzle-shaped contact piece, with the second contact piece being movable between a quench position near the first contact piece and a gap position remote therefrom. The second nozzle-shaped contact piece is held in the quench position outside the break position by the force of a spring, and in the course of the break motion it is brought from that position into the break position against the force of the spring by the switch drive via a driving member.
In both switch embodiments, as in all known double-nozzle contact systems with a bridging contact piece, the arc is initially drawn during the breaking process between the one nozzle-shaped switch element, the so-called roll-off contact piece, and the bridging contact piece. As the breaking process continues, the arc is switched from the bridging contact piece to the second nozzle-shaped contact piece. The metallic contact devices required for transferring the rated current, notable at the bridging contact piece, participate in guiding the arc, so that under the influence of the arc metal vapors get into the gap. Such vapors may impair the dielectric strength of the gas gap between the open contacts when the arc is quenched during the natural passage of the alternating current through zero.